This invention relates to an improvement in a three-point system seat belt for restraining a passenger seated on a car seat and more particularly contemplates ensuring mitigation of impact to the head and waist of the passenger and to protect him at the time of high speed collision of the car.
Various studies have previously been made in an attempt to minimize the impact energy in a seat belt and to protect a passenger, and various counter-measures have already been proposed such as those involving the construction of the belt webbing or the like. In these conventional systems, however, a greater emphasis has been placed on the shoulder belt rather than on the waist belt, partly for the purpose of coping with head-on collisions, or from the viewpoint of standards imposed on the seat belt. Hence, the energy absorption capacity of the shoulder belt has been of greater significance. In this regard, U.S. Pat. No. 3,891,272 discloses a three-point seat belt system wherein the shoulder belt is provided with overlapping sections joined together with stitching which is rupturable under progressively larger loads. The rupturing of the stitching causes the shoulder belt to extend and to absorb some of the impact energy.
As a result of analysis of actual high speed car accidents and data of collision experiments in order to trace and examine a transfer quantity to each of the head, chest, waist and femur, it has been found that though the energy absorption effect of the shoulder belt is good when the car speed is relatively low, stretch-out of the shoulder belt becomes greater as the car speed increases whereby an injury index of the upper half of the passenger's body, especially that of the head, becomes greater.